1. Field of the Invention
This invention relates to a method of manufacturing void-free laminates at low temperatures under autoclave-pressure or vacuum-bag-only-pressure. In particular, the invention relates to the use of partially impregnated prepregs in which the level of voidage in the resulting laminate is lower than that achieved by vacuum-bag-only-pressure techniques to-date.
2. Related Background Art
Composite materials consist of a matrix resin and reinforcement fibers. These materials are typically used in areas where high strength and low weight are important, for example, the aerospace and aircraft industries. Most composites used in aerospace structural applications comprise thermosetting resins and carbon fiber materials. Typically, these thermosetting resins are cured at high temperature (e.g., 250-350xc2x0 F.) and under high pressure (e.g., 85 psi) using an autoclave.
A common method of manufacturing composite materials is to lay-up a number of sheets of uncured resin impregnated filaments called prepregs on a suitable tool and subject them to heat and pressure in order to coalesce the sheets, mold them to the configuration of the mold and subsequently gel the resin. The resin is then finally cured by further heat treatment in order to fix the resulting configuration of the molded laminate.
A major problem encountered in the manufacture of thick composite parts is porosity (or voids) in the final part. Although the occurrence of voids in composite materials is not completely understood, it is believed to be, in part, due to the fact that entrapped air cannot escape from the prepregs used in the manufacture of composite materials. Campbell et al. have studied the cause of porosity in carbon fiber composites (Flake C. Campbell et al., Journal of Advanced Materials, 18-33, Jul. 1995). Void formation and growth in composite laminates is primarily due to entrapped volatiles. Void growth will potentially occur if the void pressure (i.e., the volatile vapor pressure) exceeds the actual pressure on the liquid resin (i.e., the hydrostatic resin pressure) while the resin is still a liquid. Composite parts processed under similar conditions have been found to result in significantly different voidage levels resulting in production slowdowns. Void formation seriously compromises the mechanical properties of the composite material and in many cases requires large repair costs due to rejection of parts before they can be employed.
One way in which a void-free laminate can be manufactured is to utilize an autoclave. An autoclave is capable of subjecting the laid-up prepregs to elevated temperatures and pressures so that they can readily coalesce to form a reinforced composite material. This apparatus has the attraction of being capable of supplying sufficient pressure to the resin mass that hydraulic pressure within the mass causes a significant reduction in the size of enclosed gas or vapor bubbles or completely forces them into solution depending on the level of pressure applied. If the pressure is maintained during the gelation of the resin and its subsequent cure, a void-free matrix is achieved.
However, while pressure application from an autoclave is attractive in view of its potential for providing a void-free reinforced composite part, it is nevertheless expensive in view of the high capital cost of the equipment involved. Furthermore, autoclaving is deemed undesirable when the size of the reinforced composite part is too large to be efficiently cured in such a manner. Additionally, when making reinforced composite parts at low production rates, low cost tools made of wood or low glass transition temperature polymer tools are commonly used. When these tools are used, however, composite parts can only be cured using relatively low temperatures and pressures. Thus, the use of an autoclave is not practical in these circumstances.
A cheaper alternative to autoclaving is using a process in which the laid-up prepregs are placed on a tool and then enclosed by an impervious membrane. The volume enclosed by the membrane is evacuated and the assembly heated up slowly. Ambient atmospheric pressure provides the necessary force to coalesce the prepregs to form the molded laminate and the rising temperature rate ensures that the uncured resin is initially sufficiently mobile to permit maximum consolidation and to finally permit gelling and curing of the resin at more elevated temperatures.
While pressure application using a vacuum bag is more cost effective than employing an autoclave, the resulting laminate is usually of inferior quality because of the occurrence of voids in the resin matrix. The voids are trapped in both intralaminar and interlaminar areas. Normally, the center area of the laminate is most effected compared to the edge areas of the laminate. Typically, the minimum void level of a vacuum molded fiber reinforced composite material is from about 4 to about 6 percent by volume. The state of the current commercially available low temperature, (150xc2x0 F.) vacuum-bag-only-pressure cure prepregs was reviewed. It was concluded that current technology does not permit formation of void-free unidirectional tape laminates by vacuum-bag-only-pressure and a 150xc2x0 F. cure process (Chris Ridgard, Int""l SAMPLE Symp., 147-161, 1997).
Production of void-free laminates has been attempted using both bleed curing and net curing processes. In bleed curing some of the thermosetting resin is allowed to flow out of the prepreg, carrying out trapped air and volatiles (Flake C. Campbell, Journal of Advanced Materials, 18-33, July, 1995). Although this process can produce reduced void laminates after curing, the resin content is hard to control. Thus, the final composite material does not form a reliable and consistent part. This process usually results in the formation of a thin laminate having voids due to over-bleeding of the resin.
The opposite approach is taken in net resin curing, which is a non-bleed process. All the resin remains in the cured part, offering good control of the resin content, dimension and weight of the cured parts. However, in order to produce void-free laminates it is necessary to use autoclave curing to apply sufficient pressure to force any trapped air and volatiles out of the prepregs prior to curing.
An additional approach has been developed in order to produce laminates that minimize or have substantially zero void content. WO 98/38031 discloses a method of molding a composite comprising laying alternatively on a mold first and second fiber material pre-impregnated with uncured resin, the resin content of the first layer is different from that of the second layer, the layers are enclosed within an impervious membrane which is evacuated and heat is applied to partially cure the resin to harden the material. The partially cured material may be removed from the mold and finally cured at an elevated temperature while unsupported by a mold. Although, such a process may lead to a desired material having a low void content a number of disadvantages are present in a laminate produced in such a manner. Such a process employs a high resin content first prepreg layer with normal or relatively low viscosity resin and a low resin content second prepreg layer made with relatively toughened, high viscosity resin. As a result, the employment of two prepreg layers with resins having different characteristics leads to prepregs having different tack. This complicates handling of the prepregs and necessitates that the prepregs employed are registered properly in order to provide a laminate with the desired characteristics. Additionally, utilization of prepregs having resins with different viscosities may lead to additional complications since the viscosities of the resins during storage and/or during transportation may alter. Accordingly, different conditions may have to be controlled for each resin employed, leading to additional expense and effort, in order that the resins maintain their desired viscosity characteristics.
In view of the difficulties in preparing void-free reinforced composite materials it has been desired to develop a method that allows the consistent manufacture of a void-free composite material under vacuum-pressure and low temperature cure conditions. Such a method should provide means to control the resin content, dimension and weight of the cured parts for a fiber-reinforced resin composite. Such a method would also lead to significant cost savings since the quality of the laminate could be effectively controlled and expensive autocuring equipment would not be required. Furthermore, such a method would provide a consistent method to prepare void-free laminates for prototyping and tooling in the aerospace and aircraft industries which have desired mechanical strength and low weight.
The present invention relates to a method of forming laminates in which the level of voidage in the resulting laminate is less than about 2% and may be completely eliminated.
The present invention relates to a partially impregnated prepreg comprising a fiber layer partially impregnated with a resin composition, said resin composition when heated in a vacuum envelope fully infuses into said fiber layer and cures to form a void-free laminate.
The invention also provides a method of forming a void-free laminate, comprising the steps of:
(a) enclosing a partially impregnated prepreg in a vacuum envelope, said partially impregnated prepreg comprising a fiber layer partially impregnated with a resin composition; and
(b) heating said partially impregnated prepreg under vacuum to withdraw air present in said partially impregnated prepreg and to cause said resin composition (i) to fully infuse into said fiber layer and (ii) to cure thereby forming said void-free laminate.
The method may include the step of: positioning a second partially impregnated prepreg on said partially impregnated prepreg of step (a). It is within the scope of this invention to evacuate the vacuum envelope prior to heating the partially impregnated prepreg or to simultaneously heat the prepreg while evacuating the envelope.
In a preferred embodiment the resin composition is a semi-solid at room temperature and is a liquid at cure temperatures with a viscosity from about 100 to about 1200 poise.
The invention also provides a resin composition comprising:
(a) from about 70 to about 90 weight percent of a tris(hydroxyphenyl)methane based epoxy semi-solid resin; and
(b) from about 10 to about 30 weight percent of a tetraglycidylmethylenedianiline liquid resin.
The invention further provides a resin composition comprising:
(a) from about 45 to about 65 weight percent of a glycidyl ether of phenolic novolac resin;
(b) from about 5 to about 20 weight percent of a bisphenol A epoxy semi-solid resin; and
(c) from about 10 to about 30 weight percent of a bisphenol A epoxy liquid resin.
The invention also provides a resin composition comprising:
(a) from about 20 to about 30 weight percent of a glycidyl ether of cresol novolac resin;
(b) from about 35 to about 45 weight percent of a tetraglycidylmethylenedianiline resin;
(c) from about 14 to about 26 weight percent of a bisphenol A epoxy liquid resin; and
(d) from about 0 to about 8 weight percent of a bisphenol A epoxy solid resin.
One preferred resin composition of the present intention comprises:
(a) from about 20 to about 30 weight percent of 
xe2x80x83wherein n is 2.8;
(b) from about 35 to about 45 weight percent of 
(c) from about 14 to about 26 weight percent of 
xe2x80x83wherein n is 2.5; and
(d) from about 0 to about 8 weight percent of 
xe2x80x83wherein n is 30.5.
The invention also provides a method of forming a void-free laminate, comprising the steps of:
(a) enclosing a partially impregnated prepreg in a vacuum envelope, said partially impregnated prepreg comprising a fiber layer partially impregnated with a resin composition, said resin composition comprising a glycidyl ether of cresol novolac resin; and
(b) heating said partially impregnated prepreg under vacuum to withdraw air present in said partially impregnated prepreg and to cause said resin composition (i) to fully infuse into said fiber layer and (ii) to cure thereby forming said void-free laminate.
Alternatively, the method of forming a void-free laminate could be formed by substituting the resin composition comprising a glycidyl ether of cresol novolac resin with any of the other resin compositions described above.
The void-free laminates prepared according to the present invention method may be employed to form reinforced composite materials for use in areas where high strength and low weight are important, for example, in the aircraft industry.